This invention relates generally to circuit boards and more particularly to a tri-metallic material construction for use in Etched-Tri-Metal, xe2x80x9cETMxe2x80x9d, multi-layer circuit boards.
In the field of electronics manufacturing, various additive and subtractive processes are known for constructing printed circuit boards (PCBs). Among these are the processes disclosed in U.S. Pat. No. 3,801,388 issued Apr. 2, 1974 to Akiyama et al. which is a subtractive process. U.S. Pat. No. 4,404,059 issued Sep. 13, 1983 to Livshits et al is an additive process. U.S. Pat. No. 5,738,797 issued Apr. 14, 1998 to Belke, Jr. et al is assigned to a common assignee. All three of which are incorporated herein by reference. These patents describe various additive and subtractive plating and chemical etching processes for constructing multi-layer PCBs having air bridges.
Currently tri-metallic materials as used in the manufacture of PCBs are a sandwich of aluminum as the middle layer and copper as the top and bottom layers. The layers are typically bonded together with a zinc bonding layer. Due to the anodic nature of zinc relative to copper and its position in the Electromotive Force Series, EMF series, the zinc layer is prone to corrosion.
It is a principal advantage of the tri-metallic material of the present invention to provide a bonding layer between the middle or first layer and each of the top and bottom or second and third layers that is substantially resistant to corrosion.
It is still another advantage to provide a bonding layer that has excellent adhesion properties between the middle or first layer and each of the top and bottom or second and third layers.
It is still another advantage to provide a barrier layer that has excellent properties to prevent diffusion between the middle and either or both the second and third layers.
These and other advantages will become apparent from the tri-metallic material for use in multi-layer printed circuit boards having a first metallic layer with spaced-apart top and bottom broadsides. A second and third metallic layers having spaced-apart top and bottom broadsides for overlying the first layer. A metallic bonding layer is deposited on the top and bottom broadsides of the first metallic layer that is sandwiched between the second and third metallic layers. The bonding layer bonds the second and third metallic layers to the top and bottom broadsides respectively of the first metallic layer.
In the preferred embodiment the first metallic layer is aluminum and the second and third metallic layers are copper with the bonding layer being a noble metal. In particular the bonding layer in one embodiment is immersion tin deposited on the broadside surfaces of the aluminum. A thin layer of nickel is deposited on the tin, and a copper layer is plated on the nickel layer.
If a barrier layer is desired, to prevent diffusion between layers, the process is the same as is the material of the layer. However some metals function better as a diffusion barrier than they do as a bonding layer.